A storage battery system is known which is equipped with a battery-state estimation device for estimating a deterioration state of a lithium-ion secondary battery. In order to estimate the deterioration state of a lithium-ion secondary battery, one conventional method determines presence or absence of lithium deposition. In order to determine the presence or absence of lithium deposition, in one conventional method, a lithium-ion secondary battery is discharged at a constant current; the discharge is stopped when the battery voltage is decreased to a predetermined discharge stop voltage; and based on an amount of recovery of the battery voltage after that, the presence or absence of lithium deposition is determined (see PTL 1 below).